Please find the below Post which might solve your issue here: Perform Identity Request Maintenance task does not complete Pending requests - IdentityIQ (IIQ) / IIQ Discussion and Questions - SailPoint Developer Community Forum
If the above post doesn’t help, it would really be helpful if you can share the Task config and if they are multithreaded.
Additionally, have you modified the OOTB workflows for provisioning within your environment? If yes I would suggest to use the OOTB workflows and only reference them once your modifications are made in your plans.
I would suggest the following running a cleanup operation for the access requests that were processed from the day you have encountered this issue.
Thanks,
Aman